AVICULOPECTEN FALLAX. 75 



Exterior. — The surface of both valves is covered with numerous somewhat 

 irregular, radiating, angular ribs, a finer rib often arising between two larger ones. 

 In the right valve the ribs are finer and more regular and more uniform in size, 

 and crenulated. The posterior ears are ornamented in the same way as the rest of 

 the valve, there being no real break between the valve and the posterior ear. The 

 anterior ear of the left valve is also crossed by several radiating ribs. The anterior 

 car of the right valve has its ribs interrupted by scalariform lines. 



Dimensions. — PL XII, fig. 10, measures — 



Antero-posteriorly .... 33 mm. 



Dorso-ventrally . . . .32 mm. 



Localities. — Scotland : the Calciferous Sandstone series of river Esk, Glen- 

 cartholm, Dumfriesshire. 



Observations. — This species occurs abundantly and in all stages of growth, in a 

 series of shales at Glencartholm, on the banks of the river Esk, with a very 

 peculiar fauna, which, as a group, has not yet been recognised elsewhere. 

 Fortunately, specimens showing the right and left valves still joined have been 

 obtained. The peculiar nature of the posterior ears of the valve has a good deal 

 in common with that which obtains in A. plicatus, Sow., sp., the line of depression 

 marking the ear from the valve being very oblique, and reaching the posterior 

 margin low down. The ribs in A. plicatus are much finer, more numerous and 

 regular; and the shape of the valve is more nearly circular and less gibbose than 

 in this new species. 



Aviculopecten fallax, M'Coy, sp., 1844. Plate XVII, figs. 28—30. 



Pecten fallax, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 92, pi. xiv, 



figs. 2, 2 a. 



Specific Characters. — Shell of only medium size, triangularly suborbicular, very 

 moderately convex, the right valve less so than the left. Margin regularly rounded. 

 Hinge-line straight and long. Umbones central, small, pointed, not raised, gibbose 

 in the left valve, but flattened and almost obsolete in the right. Ears well defined 

 on both sides; the posterior prolonged along its upper border and pointed 

 posteriorly; the anterior ear in the right valve depressed and narrow, separated 

 from the valve by an elongate slit for the byssus, in the left valve triangular, its 

 margin falciform, not reaching as far forwards as the anterior margin of the valve. 



Interior. — Smooth, the adductor muscle-scar shallow, rounded, placed high up 

 in the valve and posterior to the middle line. 



Exterior. — The right valve is ornamented with several somewhat irregular. 

 coarse, radiating ribs, few and almost obsolete near the umbo, and fresh ribs arise 



11 



